The present invention relates to a magnetic recording medium, and more particularly to a magnetic recording medium capable of perpendicular magnetization recording.
Magnetic recording utilizing perpendicular magnetization has been studied in recent years for increasing the recording density of magnetic recording mediums. The magnetic recording medium used in this type of magnetic recording is composed of a base made of a non-magnetic material, a first magnetic layer made of a soft magnetic material coated on a surface of the base, and a second magnetic layer having perpendicular anisotropy and coated on a surface of the first magnetic layer. Desired data can be recorded in a high density by magnetizing the second magnetic layer in a transverse direction thereof or a perpendicular direction.
FIG. 1 of the accompanying drawings shows in cross section such a magnetic medium for perpendicular magnetization and a recording head used for perpendicular magnetization. The base 1 is made of a nonmagnetic material such as synthetic resin such as polyester or polyimide or an aluminum sheet subjected to anodic oxidization. The first magnetic layer 2 of a soft magnetic material and the second magnetic layer 3 are successively formed on the surface of the base 1 to thereby construct the perpendicular-magnetization recording medium in the form of a tape or a disk.
The recording head is composed of a main magnetic pole 5 and an auxiliary magnetic pole 6 which are disposed one on each side of the magnetic recording medium. The main magnetic pole 5 has a thickness of about 1.mu. and is deposited as by sputtering on one surface of a support body 4 made of a nonmagnetic material such as glass or polyimide. A predetermined number of turns of an exciting coil 7 are wounded around the auxiliary magnetic pole 6. When the main magnetic pole 5 is excited from the auxiliary magnetic pole 6 by passing a signal current to be recorded through the exciting coil 7, a strong perpendicular magnetic field is generated in the vicinity of the tip end of the main magnetic pole 5 for magnetically recording the signal on the second magnetic layer 3 confronting the main magnetic pole 5.
It is known that the frequency characteristics particularly in the high frequency range can be improved by reducing the distance l between the first and second magnetic layers 2, 5 as small as possible. Therefore, the trend in the industry is toward a thinner configuration of the second magnetic layer 3.
With the conventional magnetic recording mediums, the first magnetic layer 2 is made of an alloy of iron and nickel or permalloy composed of such an alloy with copper and molybdenum added thereto, and the second magnetic layer 3 is made of an alloy of cobalt and chromium. Cobalt which is one of the constituents of the second magnetic layer 3 is of a hexagonal close-packed lattice (h, c, p) structure having a large crystallomagnetic anisotropy in the direction of the c-axis. By adding chromium to cobalt, the saturation magnetization of the layer is lowered to increase the ease with which the c-axis can be oriented perpendicularly to the surface of the layer. For this reason, the cobalt-chromium alloy is used as the material of the second magnetic layer 3.
However, the magnetic layer of cobalt-chromium alloy has a relatively small, about 0.6, ratio Br (.perp.)/Br (.parallel.) of a perpendicular residual magnetic flux density Br (.perp.) to a horizontal residual magnetic flux density Br (.parallel.) (in the plane of the layer), the ratio being hereinafter referred to as a "perpendicular residual magnetic flux density ratio"), and an anisotropic magnetic intensity Hk of about 2400 Oe, magnetic characteristics which are not sufficiently satisfactory. The above tendency manifests itself where the film thickness of the magnetic layer is 0.3 .mu.m or smaller.
To improve the magnetic characteristics, it has been proposed to add a third element such as tungsten (W), molybdenum (Mo), or a rhenium (Re) to the cobalt-chromium alloy. However, the third element fails to be sufficiently effective, and particularly cannot increase the perpendicular residual magnetic flux density ratio above 1.